1. Field of the Invention
The present invention relates to an automatic focusing apparatus for a video camera.
2. Description of the Prior Art
Various automatic focusing apparatus for video cameras have been proposed and put to practical use. Among those known automatic focusing apparatus, an automatic focusing apparatus of a system which uses the output signals of the image pickup element of the video camera, namely, an automatic focusing apparatus of a so-called "In-focus control servo system" is disclosed, for example, in Ishida et al., "Automatic Focusing of a Television Camera on an In-Focus Control Servo System", NHK Gijutsu Kenkyu Hokoku, Vol. 17, No. 1, p.21.
An example of the conventional automatic focusing apparatus which uses image signals for focusing the lens system of a video camera will be described hereinafter.
In this conventional automatic focusing apparatus, the output signal of an image pickup element is transmitted through a high-pass filter to a rectifier. The output signal of the rectifier is applied to a differential hold circuit. The output signal of the differential hold circuit is amplified by a servo amplifier. A motor for driving a focusing mechanism for focusing a lens system is driven by the output of the servo amplifier.
In operation, the image of an object focused on the image pickup element by a lens system is converted into a corresponding electrical image signal, and then the electrical image signal is supplied to the high-pass filter. The rectifier rectifies the high-frequency component of the electrical image signal filtered by the high-pass filter, and then provides an output signal (hereinafter referred to as a "focus voltage signal") proportional to the high-frequency component of the electrical image signal. The differential hold circuit holds the focus signal for a predetermined time period to obtain a difference between the successive focus signals, and then provides a focus voltage variation signal representing the rate of variation of the focus voltage with time. The focus voltage reaches a maximum when the lens system is focused and decreases with the deviation of the lens system from a position where the image is in focus, and hence the focus voltage curve is a generally upward convex curve having a peak point. When the output signal of the differential hold circuit is positive, the focus voltage is ascending along the focus voltage curve, and hence the motor is driven so that the the focusing ring is turned further in the same direction and the focusing ring is stopped when the focus voltage variation signal is equal to zero. When the output signal of the differential hold circuit is negative, the focus voltage is descending along the focus voltage curve, and hence the motor is driven so as to turn the focusing ring in the reverse direction. Thus, the output of the differential hold circuit is supplied after being amplified by the servo amplifier to a motor driving circuit to drive the motor so that the focusing ring is turned accordingly for focusing.
The conventional automatic focusing apparatus of such a constitution, however, executes a signal processing operation only with respect to the horizontal direction of an image plane, and hence, in some cases, the automatic focusing apparatus is unable to focus the taking lens system on a horizontally invariable object, such as a horizontal line.
In viewing an insufficiently illuminated object or an object having a low reflectance, only a focus voltage of a low level is available, which is insufficient for a focusing operation. Furthermore, the focus voltage curve is also dependent on the f number and focal length of the lens system. For those reasons, the automatic focusing apparatus is unable to function stably for a focusing operation.
Thus, the conventional automatic focusing apparatus is unable to function stably because the focusing operation of the conventional automatic focusing apparatus is susceptible to the spatial frequency component of the object, the condition of the object or the viewing conditions.